1. Field of the Invention
The present invention generally relates to control systems for injection molding machines and methods for controlling the injection molding machines. More particularly, the present invention relates to a control system for an injection molding machine performing a molding cycle including a filling process and a hold press process following the filling process, and a method for controlling the injection molding machine.
2. Description of the Related Art
First, an explanation will be given of a molding cycle of an injection molding machine, comprising a plasticizing process, a metering process, a filling process and a hold press process in a case of a motor-driven injection molding machine as an example.
(1) Plasticizing Process/Metering Process
In the plasticizing and metering processes, a servo-motor for screw rotation rotates the screw. The screw is located within a heating cylinder. Resin is fed from a hopper to the rear portion of the screw in the heating cylinder. By the rotation of the screw, the resin provided to the rear portion of the screw from the hopper is melted and a given amount thereof is fed to the tip end of the heating cylinder. During this time, the screw is retracted while being subjected to a back pressure, namely the pressure of molten resin accumulating at the tip end of the heating cylinder.
An injection shaft is directly connected to a rear end portion of the screw. The injection shaft is rotatably supported by a pressure plate through a bearing. The injection shaft is driven in the axial direction by a servo-motor for injection that is supported on the pressure plate. The pressure plate is driven through a ball screw by a servo-motor for injection to advance and retract along guide bars. The foregoing back pressure of molten resin is detected by a load cell in a manner described later. The detected value of the load cell is controlled by a feed-back control loop for pressures.
(2) Filling Process
Then, in the filling process, driving of the servo-motor for injection causes the pressure plate to advance to fill molten resin into a mold with the screw tip end as a piston. Resin pressure at the screw tip end at this time is detected as an injection pressure.
At the end of the filling process, the molten resin fills a cavity of the mold. At that point, the advancing motion of the screw causes switching of velocity control to pressure control. Such switching of velocity control to pressure control is referred as a V-P switching and affects the quality of the resultant molded article.
(3) Hold Press Process
Following the V-P switching, the resin within the cavity of the mold is allowed to cool under a preset pressure. This process is referred to as a hold press process. In the hold press process, resin pressure is controlled with a feed-back control loop as in the above-mentioned back pressure control.
In an injection apparatus of the injection molding machine, when the process (3) is completed, the control system goes back to the process (1) and shifts to the succeeding molding cycle. In a mold clamping apparatus of the injection molding machine, concurrently with the process (1), the mold is opened to permit an ejector mechanism to discharge a molding product having been cooled and solidified, and then the mold is closed for the process (2).
Meanwhile, for injection molding, measurement precision greatly depends on the profile of the resin pressure in the filling and hold press processes. For example, in a case of manufacturing a thin-walled molded article, due to differences of the resin pressure just after the V-P switching in particular, molding defects phenomena such as a short shot, sink, flash, warpage, and the like may be generated every so often.
There are V-P switching position settings, injection speed settings, hold press settings and the like as molding conditions in order to prevent the above mentioned molding defects phenomena. An operation for the above mentioned settings is called a molding condition determination. The molding condition determination is complex because the above mentioned settings influence each other. Hence, only the “hold press settings” and “time settings” are used as the molding conditions in the conventional hold press process and several values of pressure are set as the hold press settings.
For example, in a case where the resin pressure at the time of V-P switching is high, if the hold press pressure that makes the resin pressure low is set as a hold press setting value, the screw retracts. However, if the retracting speed is not controlled, as described later, there are problems in that the molded article is adversely affected.
FIG. 1 is a view for explaining the filling process and hold press process of the molding cycle when a conventional method for controlling is applied.
More specifically, FIG. 1-(a) shows a profile of injection speeds in the filling process and hold press process of the molding cycle when a conventional method for controlling is applied; FIG. 1-(b) shows a profile of the resin pressure in the filling process and hold press process of the molding cycle when a conventional method for controlling is applied; FIG. 1-(c) shows a profile of a screw position, that is the distance between the tip end of the screw and the end part at the nozzle side of the heating cylinder, in the filling process and hold press process of the molding cycle when a conventional method for controlling is applied.
In FIG. 1-(a), positive injection speed represents an advancing of the screw and negative injection speed represents a retracting of the screw. As shown in FIG. 1-(b), for convenience, only a first segment of pressure setting in the hold press process is shown as an example. Control for the hold press process is performed based on setting the hold press pressure Ph1 and time t1.
As the injection begins, the screw advances as shown in FIG. 1-(a), and the distance between the tip end of the screw and the end part at the nozzle side of the heating cylinder is shortened, so that the resin fills the cavity of the mold.
There is a case where the volume of the resin in the cavity increases when the molten resin compressed by a gate of the screw tip end is injected into the cavity of the mold. In this case, even if the pressure of the screw head part is changed in order to change the hold press pressure in the hold press pressure process, the resin in the cavity is seldom influenced or reacts excessively.
In a case where the resin pressure at the time of the V-P switching is high, when the first hold press setting value is set so as to reduce the resin pressure, the screw retracts. That is, a retracting speed of the screw is generated. Particularly, in a case where there is a big difference between the resin pressure at the time of the V-P switching and the first hold press setting value Ph1, as shown in a part surrounded by a dotted line in FIG. 1-(a), the retracting speed of the screw becomes high. If the retracting speed of the screw becomes high, a negative pressure is applied to the molten resin. As a result of this, an adverse effect on the molten resin, such as the generation of a void in the molten resin, may be invited.